1. Field of the Invention
The present invention relates to a bonding material employing metal particles with a diameter of 1 nm to 50 μm as a main bonding agent, and also relates to a semiconductor device on which a bonding process is performed by the use of the bonding material.
2. Description of the Related Art
It is known that, when a diameter of a metal particle decreases to 100 μm or less and the number of constituent atoms decreases, the ratio of a surface area to a volume of the metal particle sharply increases so that the melting point or sintering temperature of the metal particle is considerably lowered compared to that of a bulk-type metal. It has been studied to use a metal particle with a diameter of 1 nm to 100 nm as a bonding material by utilizing the low-temperature sinterability (refer, for example, to Japanese Patent Application Laid-Open (JP-A) No. 2004-107728). JP-A No. 2004-107728 discloses that a bonding process is carried out by performing a heating process to decompose an organic material and sinter metal particles to each other by using a bonding material prepared by coating a core made of metal particles having an average diameter of 100 nm or less with the organic material. In this bonding method, since the bonded metal particles change to a bulk metal and simultaneously the bonded metal particles are bonded through the metallic bonding at a bonding boundary, they have extremely high heat resistance, reliability, and high heat dissipation property. Furthermore, while a lead-free solder has been demanded in the connection of electronic parts, etc, lead-free materials as a substitute for high-temperature solders have not been developed yet. In mounting electronic parts, it is indispensable to use hierarchical soldering, and therefore there is an ongoing demand for the advent of a material in place of the high-temperature solders. Accordingly, it is also expected that the bonding technique of the present invention can provide a material capable replacing the high-temperature solder.
The present inventors have made a study on the bonding material using metal particles with an average diameter of 100 nm or less as a main bonding agent which is disclosed in JP-A No. 2004-107728. As a result, it has been found that while the bonding material can provide satisfactory bonding strength to a mating electrode formed of gold (Au), silver (Ag), or palladium (Pd) as a material to be bonded, no sufficient bonding strength can be obtained to aluminum (Al) that is popularly used in the mounting of the semiconductors. FIG. 8 shows evaluation results of the bonding strength of each electrode material. A bonding process was performed on Au, Ag, Al, Ni, and Cu electrodes in atmospheric air under conditions that a bonding temperature and a pressure are maintained constantly, e.g., 250° C. and 1.0 MPa, respectively, and a bonding material employs silver particles with an average diameter of 10 nm coated with an amine-based organic material. In FIG. 8, the ordinate shows a shearing strength, which is normalized with respect to the value of the Ag electrode. As a result, it can be observed that satisfactory bonding strength can be obtained in the Au and Ag electrodes but no sufficient bonding strength can be obtained in the Ni and Cu electrodes when the electrodes are bonded in atmospheric air. Moreover, it could be found that it was utterly impossible to bond the Al electrode.
The organic material coated on ultra fine particles disclosed in JP-A No. 2004-107728 is a material vanished only by heating in atmospheric air and it is not suitable for the bonding of Al which is highly susceptible to oxidation although the organic material is effective for an electrode which is less susceptible to oxidation.
In a case of bonding electronic parts constituting a semiconductor device using a bonding material employing ultra fine metal particles as a main bonding agent, it is necessary to ensure electrical conductance. Furthermore, relaxation of thermal strain and heat conductivity are also required for the bonding material. In addition, an AL electrode that is being widely used should be able to be bonded. These requirements are also applied to the bonding of a conductor and an Al member such as an Al wire and an Al base plate (heat sink plate) forming an electronic part, while not limiting to the bonding of the Al electrode and the semiconductor device.